Add a way to ignore JIT stencil digest

Feature or enhancement

Proposal:

In Fedora, we use prebuilt JIT stencils on some of the Fedora versions/editions.

When autoconf was updated from 2.72 to 2.73 in Fedora Rawhide, we observed a change in the checksum:

--- /builddir/build/SOURCES/Python-3.15.0a7-x86_64-debug-jit_stencils.h 2026-03-10 00:00:00.000000000 +0000
+++ build/debug/jit_stencils-x86_64-unknown-linux-gnu.h 2026-03-31 21:12:53.391262973 +0000
@@ -1,4 +1,4 @@
-// 5e1dc452d67c20147c4ea13e692c2d3e7b0b47b24f8847354c58988c7ffa02bc
+// c783e3946f774d4d83e8fdcca35678ec77006e1b96b12661fcbd3ee680d10e94
 // $ python3.14 /builddir/build/BUILD/python3.14-3.15.0_a7-build/Python-3.15.0a7/Tools/jit/build.py x86_64-redhat-linux-gnu --output-dir . --pyconfig-dir . --cflags= --llvm-version= --debug

The autoconf-generated pyconfig.h file is part of the checksum input:
https://github.com/python/cpython/blob/v3.15.0a8/Tools/jit/_targets.py#L75

My understanding is that the reason for this checksum check is to prevent outdated JIT stencils when Python is re-configured with different options (or when the stencils generation code is updated).

As suspected, the change in checksum was caused by a change in pyconfig.h:

--- pyconfig.h
+++ pyconfig.h
@@ -1882,6 +1882,10 @@
 #ifndef _ALL_SOURCE
 # define _ALL_SOURCE 1
 #endif
+/* Enable extensions on Cosmopolitan Libc. */
+#ifndef _COSMO_SOURCE
+# define _COSMO_SOURCE 1
+#endif
 /* Enable general extensions on macOS.  */
 #ifndef _DARWIN_C_SOURCE
 # define _DARWIN_C_SOURCE 1

This is expected, see https://cgit.git.savannah.gnu.org/cgit/autoconf.git/tree/NEWS?h=v2.73#n99

tl;dr When a new autoconf version changes the output of pyconfig.h, the checksum changes as well.

We cannot update the pre-built stencils with the new checksum because that would make the build fail with older autoconf.

Hence, I am proposing we have a way to instruct the Tools/jit/_targets.py tool to ignore checksum mismatch. We would then use this option in Fedora.

For example, ./configure --enable-prebuilt-jit-stencils.

I will open a draft PR for consideration. Thanks.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

What we do in Fedora was previously discussed in https://discuss.python.org/t/building-the-jit-with-pre-built-stencils/91838

Linked PRs

• gh-148647

Comienza con Tools/jit/_targets.py y la propuesta existente --enable-prebuilt-jit-stencils; después, revisa el PR preliminar vinculado gh-148647. Verifica cómo la suma de comprobación incluye pyconfig.h y define qué comportamiento debe producirse cuando los stencils precompilados provienen de una versión diferente de autoconf. Se considera terminado cuando Fedora puede excluirse del fallo por discrepancia sin debilitar la protección predeterminada.